Reductive precipitation of metal ions from solution by hydrogen is an electrochemical process that has been practised commercially in nickel recovery for many years. Variants of the method have been described by Sherritt Gordon Mines and Amax. In commercial processes, nickel is readily recovered from a nickel sulphate complex by hydrogen reduction, for example in the reaction:NiSO4+H2→Ni0+2H++SO42−
By this reaction, hydrogen ions are released and unless they are neutralised, the pH will decrease, inhibiting further reaction. Neutralisation of the hydrogen ions may be achieved by the addition of ammonia, which combines with the nickel sulphate to form a nickel ammine sulphate complex. This nickel ammine sulphate complex may be reduced by hydrogen, and in the case of nickel diammine sulphate, the product formed will be a nickel product and ammonium sulphate, as shown by the equation:Ni(NH3)2SO4+H2→Ni0+(NH4)2SO4 
The process embodied by this equation will take place at a constant pH and therefore does not suffer the inhibition of further reaction which can occur without neutralisation.
Although favourable thermodynamics exist for nickel to be reduced by hydrogen at room temperature, in practice, elevated temperatures and pressures are required to achieve favourable kinetics. Metal complexes such as the ammines of nickel decrease in stability with increasing temperature and often hydrolysis results in precipitation of salts and hydroxides. This problem is minimised by the presence of high concentrations of non-reactive salts such as ammonium sulphate.
Therefore, commercial systems operate with an NH3:Ni ratio of 2:1 in the presence of high strength ammonium sulphate at elevated temperatures and pressures to achieve favourable kinetics, while minimising the precipitation of salts and hydroxides. As illustrated above, ammonium sulphate is a by-product of the reaction, which must be bled off and recovered for economic reasons.
By application of the Nernst equation, it can be shown that the potential of the hydrogen reduction system is a function of the pH of the solution and the activity of the gaseous hydrogen dissolved in the solution. This may be demonstrated as follows:At 25° C. E=0.0591 pH−0.0296 log PH2 
In the case of a nickel sulphate solution in which nickel ions have a molal activity of 1×10−4 in conjunction with a PH2 of one atmosphere, we find:
  pH  =            0.368      0.059        =    6.2  
This is the equilibrium pH and in this example, at pH values of greater than 6.2, there would be a thermodynamic tendency for the nickel complex to be reduced from a solution of nickel having 1×10−4 molal activity.
Existing aqueous hydrogen reduction processes for nickel are based on the use of ammonium sulphate liquors as outlined above. Processes have been described in the literature for hydrogen reduction of nickel from basic nickel carbonate slurries in ammonium sulphate solution which was briefly used commercially in the Phillipines, and also hydrogen reduction of nickel hydroxide slurries in ammonium sulphate solutions (T Saarinen et.al. “Pressure reduction of nickel by hydrogen from hydroxide slurries”, Hydrometallurgy 43 (1996) and International patent WO 01/348959, May 2001.
An alternative process to recover a metallic nickel product from nickel ammine carbonate solutions at similar ratios of ammonia to nickel in a similar manner to the recovery from the ammonium sulphate system above, has been described by W. Kunda et. al “Low density nickel powder by hydrogen reduction from the Aqueous Ammonium Carbonate system” Planseeberichte Fur Pulvermetallurgie, 1964.
However, there are no known commercial processes where nickel is recovered by hydrogen reduction based on high strength ammonia solutions particularly strongly ammoniacal ammonium carbonate or strongly ammoniacal ammonium sulphate solutions.
Australian Patent AU 605867 in the name of Queensland Nickel Pty Ltd (AU 605867), the entire disclosure of which is incorporated herein by reference, describes a process for the recovery of nickel in an ammoniacal solvent extraction (ASX) system. In the process described in this patent, nickel is extracted from an ammoniacal liquor with an organic reagent to create a nickel loaded organic phase, and is then stripped from the nickel loaded organic phase with a strongly ammoniacal ammonium carbonate solution leading to the formation of a nickel ammine carbonate complex. The nickel is recovered from this complex by known art.
The above discussion of documents, acts, materials, devices, articles and the like is included in this specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed in Australia before the priority date of each claim of this application.
An aim of the present invention is to demonstrate a process by which nickel, which has been extracted from an ammoniacal liquor with an organic reagent to create a nickel loaded organic phase and then stripped from the nickel loaded organic phase with a high strength ammonia solution, can be recovered from the high strength ammonia solution by hydrogen reduction. The invention is particularly applicable to a process wherein the high strength ammonia solution is a strongly ammoniacal ammonium sulphate solution leading to the formation of nickel ammine sulphate complexes in solution, or a strongly ammoniacal ammonium carbonate solution leading to the formation of nickel ammine carbonate complexes in solution. The strongly ammoniacal ammonium sulphate solution may be used in a similar manner to the ammoniacal ammonium carbonate solution used to strip nickel from the loaded organic phase in the process described in Australian patent AU605867 discussed above.
A further aim of the present invention is to demonstrate a process which integrates hydrogen reduction of nickel ammine carbonate complexes in nickel solvent extraction process solutions such as that described in AU 605867.
A further aim of the invention is to demonstrate a process which integrates hydrogen reduction of nickel ammine sulphate complexes in nickel solvent extraction solutions produced by stripping from the nickel loaded organic phase with a strongly ammoniacal ammonium sulphate solution.
A further aim of the invention is to demonstrate a process for hydrogen reduction of nickel ammine carbonate complexes in strongly ammoniacal ammonium carbonate solutions.
A further aim of the invention is to demonstrate a process for hydrogen reduction of nickel ammine sulphate complexes in strongly ammoniacal ammonium sulfate solutions.